Engines produce a variety of pollutants, including nitrogen oxides (nitric oxide or NO and nitrogen dioxide or NO.sub.2, both of which may be referred to as NOx). The NOx and unburned hydrocarbons often are considered as the worst contributors to pollution of city air. Automobile gasoline engines currently include a CCD (catalytic converter device) containing a catalyst of the platinum group, and connected through an elongated conduit to the exhaust valves of the engine cylinders. The platinum based CCD's cause a reaction between NOx and carbon monoxide, to produce nitrogen and carbon dioxide, both being substantially benign. Such CCD's are effective only when heated to a temperature of at least 350.degree. F. to 400.degree. F., and generally operate at a temperature of about 700.degree. F. or more when the engine has been completely heated after several minutes of running time.
Gasoline engines are run at stoichiometric conditions or with a few percent too little air to minimize the production of NOx. Although the engine could be run lean wherein additional air was introduced into the cylinder, to reduce unburned hydrocarbons, this would result in preferential reaction of oxygen with carbon monoxide, which hampers the reaction of carbon monoxide with nitrous oxide in the catalyst. Even with stoichiometric operation and the CCD's, significant amounts of NOx are released, and proposed legislation will require further reductions in the future. A practical system which further reduced NOx emission, and which even allowed further reductions in unburned hydrocarbon emissions, and which could be utilized with currently available platinum group-based catalytic converters, would be of considerable value. If such a system reduced oxides of nitrogen emissions, it could enhance the acceptability of diesel engines which, though highly efficient, have become unacceptable because of high emissions of oxides of nitrogen.
The present invention reduces pollution by injecting ammonia into the conduit that couples the fuel-burning cylinders or other power-generating portion of an engine, to the catalytic converter. The use of ammonia injection to remove NOx has been proposed in the past. U.S. Pat. No. 3,449,063 by Griffing et al describes the injecting of ammonia into engine exhaust gases, to flow through a copper oxide or copper oxide-palladium catalyst. U.S. Pat. No. 3,599,427 by Jones describes an engine with two catalyst stations, the first being a copper oxide catalyst which removes carbon monoxide and hydrocarbons, and with the second catalyst being of the type which includes a noble metal such as platinum. After the exhaust gases pass through the copper oxide catalyst, they are cooled to about 700.degree. F., before ammonia is injected into the exhaust gases to pass with them through the second catalyst containing noble metal. The systems of these patents require considerable modification of an engine, both requiring the addition of a copper oxide based catalyst operating at a high temperature and not efficiently using the injected ammonia. Applicant's system efficiently uses injected ammonia, and can be easily used with present engine systems or easily retrofitted to existing engines.